This study represents one portion of a major program in our laboratory whose objectives are to understand the interaction and mechanisms of action involved in the selective radiosensitization of neoplastic tissues by hyperthermia. The ultimate goal of the program is to utilize this knowledge in the treatment of cancer. The objective of the present proposal is to examine the relationship between the fractionation of treatments and the Radiation Enhancement Factor of Hyperthermia (REF-Ht) on a transplanted mouse tumor. It is proposed that quantitative studies be carried out using the Meth-A tumor grown in BALB/c male mice by varying the numbers of treatments from (1 to 5) and the days between treatments from 1 to 4. The treatments will be local tumor hyperthermia (42.5 plus or minus, 5 degrees centigrade for 15 minutes) and/or radiation (total dose of 1800 Rads). From routine tumor measurements taken after treatment, relative total surviving tumor volumes can be obtained for calculation of the REF-Ht for each fractionation regimen. Comparisons of the REF-Ht will help establish optimal time-dose fractionation schemes for the combined use of heat and radiation in the treatment of human cancer.